Radio frequency amplifiers with compensation signal to accommodate changes in the conduction point due to temperature changes

ABSTRACT

A radio-frequency amplifier incorporating at least one amplifing device comprises means for applying to the device a signal related to changes in the radio frequency power handled by the device, thereby to compensate for changes in the conduction point of the device due to temperature changes within the structure of the device resulting from changes in the radio frequency power handled by the device.

FIELD OF INVENTION

The present invention relates to radio frequency amplifiers and moreparticularly, but not exclusively, to radio frequency amplifiers for usein television transmitting apparatus.

BACKGROUND TO THE INVENTION

The devices used in such amplifiers, which may be semiconductor devicessuch as transistors, or therminoic tubes, may be required to handlelarge radio frequency (RF) currents which may fluctuate, for examplewith the degree of modulation of the RF signal being amplified.

It is an unfortuante feature of such devices that their internaltemperature will vary as a function of the average RF power handled, dueto the heating effect of the RF current. The resulting changes ofinternal temperature produce corresponding changes in the condustionpoint of the device and therefore related changes of gain.

For example, in a silicon transistor, the base-emitter voltage necessaryto initiate collector current flow, Vbe, changes by 2.1 mV for eachdegree centigrade change in the temperature of the silicon structure ofthe device.

Radio frequency amplifiers in television transmitters are commonly runin Class AB mode, in which a degree of non-linearity in thepower-in/power-out characteristic of the amplifier is tolerated at highsignal levels. However to achieve optimum performance from such Class ABamplifiers, the biassing level of the amplifying device is critical. Itwill be appreciated that variation in the conduction point of thedevice, resulting from internal temperature changes caused by radiofrequency Dower level variation can give rise to unwanted and varyingdistortion of the output signal from the device, as the average radiofrequency power level varies.

In a radio frequency amplifier within television transmitting apparatusthe problem is particularly severe as the amplitude of the modulatedradio frequency signal and hence the average radio frequency powerhandled by the device can undergo rapid swings as the signal varies fromlevels corresponding to white (minimum power) to black (maximum power).The effect of resulting non-linearities on the transmitted vision signaldue to thermal changes within the device can be most noticeable.

The thermal changes associated with the internal heating of the deviceare rapid, and occur in the internal structure of the device. It istherefore not possible to monitor and correct for such changes by meanssuch as an externally attached temperature sensor forming part of acompensation arrangement, such as may be used with radio frequency poweramplifying devices to correct for relatively long-term thermal changesdue to ambient operating conditions.

It is thus an object of the present invention to obviate at least inpart the deleterious effect of such rapid internal temperature changesdue to variations in radio frequency power level, on the performance ofdevices employed in radio frequency amplifiers.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a radio-frequency amplifierincorporating at least one amplifing device comprises means for applyingto the device a signal related to changes in the radio frequency powerhandled by the device, thereby to compensate for changes in theconduction point of the device due to temperature changes within thestructure of the device resulting from changes in the radio frequencypower handled by the device.

The compensation signal related to the changes in the radio frequencypower handled by the device may be derived from a sensing element in thecircuit of the device.

The sensing element may be connected in the principal current path ofthe device, and may be a resistor.

Alternatively the compensation signal may be derived from elsewhere in asignal chain of which the amplifier forms part, either from a point inthe signal chain prior to the amplifier, or from a point in the signalchain following the amplifier.

The compensation signal may be applied to a biassing circuit of thedevice, preferably by means of a signal filter the time constant ofwhich is related to the internal thermal time constant of the deviceassociated with changes of power level therein.

The time constant of the signal filter is preferably substantially thesame as the internal thermal time constant of the device.

The compensation signal may also be applied to a further device ordevices in the same signal chain having substantially the same internalthermal time constant as the first mentioned device.

The invention also lies in signal transmitting apparatus when providedwith a radio frequency amplifier embodying the invention, typically atelevision signal transmitting apparatus.

Embodiments of the invention will now be described solely by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is an outline circuit diagram of an RF amplifier in accordancewith the prior art;

FIG. 2 illustrates the power transfer characteristics of the amplifierof FIG. 1 with differing biassing conditions;

FIG. 3 is an outline diagram of the RF amplifier of FIG. 1 modified inaccordance with and illustrating the principles of operation of theinvention;

FIG. 4 is a block diagram of the final stages of a televisiontransmitter employing an RF amplifier in accordance with the invention;and

FIGS. 5(a) and 5(b) are the essential parts of the circuit diagram of anRF amplifier in accordance with the invention, employed in a televisiontransmitter.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, an RF amplifier comprises an amplifying device,transistor 10, provided with a conventional biassing circuit comprisingtransistors 12 and 14 and resistors 16 and 18.

An RF signal applied to the base of transistor 10 via lead 20 isamplified by the device and an amplified output derived via lead 24 froma load 22 in the collector circuit of the device.

The quiescent operating condition of the amplifier is adjusted by meansof resistor 18 to provide a bias upon its base to give the amplifier atransfer characteristic such as is illustrated at 30 in FIG. 2, whichprovides a linear response at low and medium power inputs but with anincreasing but tolerable degree of distortion at higher power inputs.This type of operation is typical of Class AB operation.

The effect of a change of conduction point of the device is shown byreference to characteristics 32 and 34 of FIG. 2. If the quiescentcurrent in the collector circuit of the device is too high, the transfercharacteristic 32 results, with a non-linearity producing excessive gainat low power input levels. If the quiescent current is too low, thetransfer characteristic 34 results with a non-linearity producing lowgain at low power input levels.

In an RF carrier modulated with a TV video signal, low power levelsequate to white signal and high power to black signal and sync pulses,with colour-related components of the signal at intermediate powerlevels. The importance of maintaining linearity (Class A operation) atlow and medium power levels will thus be apparent, as will the abilityto tolerate a limited degree of distortion (Class B operation) at highpower levels, corresponding to black and sync pulse levels.

In an amplifier such as illustrated in FIG. 1, operation of the device10 with an applied radio frequency signal produces internal heating ofthe device as a function of the RF power level handled. The heatingoccurs at the silicon surface of the device and produces a change inVbe, the base-emitter voltage of the transistor at which conductioncommences, which causes movement of the transfer characteristic of theamplifier from the ideal, giving rise to non-linearities at lower powerlevels and thus unacceptable distortion of a video-modulated RF signal.

In the RF amplifier in accordance with the invention shown in FIG. 3,the amplifier of FIG. 1 is modified by the provision of a feedbackcircuit comprising current-sensing resistor 40 in the collector circuitof transistor 10, inverting amplifier 42, low-pass filter 44 andnon-inverting buffer amplifier 45.

In operation, a voltage directly related to the modulation level of theRF current in the collector-emitter path of transistor 10, and hence tothe RF power level in the device, is developed across resistor 40. Asthe collector current increases or decreases with changes of RF powerlevel in transistor 10, so the voltage across resistor 40 increases ordecreases. This voltage is inverted by the inverting amplifier 42 andfed to the biassing circuit of transistor 10 thereby changing thebiassing level upon transisitor 10 to compensate for changes in Vbe dueto RF power-related internal heating in transistor 10.

The filter 44 determines the rate at which the bias upon the base oftransistor 10, is changed by the compensation signal derived from RFpower level changes in that device. The rate will be a function of theinternal thermal characteristic of transistor 10, and will preferably bethe same or substantially the same as the internal thermalcharacteristic of the transistor. Because the internal thermalcharacteristics of a device are not generally made known by devicemanufactures, and as different devices will have substantially differentinternal thermal characterisitcs, the desired rate of change of bias dueto the compensating signal has to be determined and set empirically, forexample by running the device with a succession of differing powerlevels within the expected dynamic operating range of transistor 10 andadjusting critical component values in filter 44 in a known manner toprovide a compensating bias to the base circuit transistor 10 tomaintain the bias level substantially constant.

The amplifier described in FIG. 3 may be employed as a driver amplifierin the final stages of a TV transmitter, the function of the amplifierbeing to provide the final drive to a series of parallel RF outputstages. Such an arrangement is illustrated in outline in FIG. 4.

RF driver amplifier 50, fed with a modulated RF signal from modulator 52via splitter 54, incorporates a feedback bias compensation arrangementin accordance with the invention, sensing resistor 56 in the collectorcircuit of transistor 58 developing a voltage related to the collectorcurrent, and hence the RF power level in transistor 58, which is fedback via amplifier 60 and low pass filter 62 to the biassing circuit oftransistor 58 to compensate for internal temperature changes intransistor 58 due to RF power level changes. At the same time thecompensating bias from the feedback circuit of transistor 58 is fed vialead 64 to the biassing networks of RF amplifying transistors of thesame type as transistor 58, in the parallel output amplifier stages 66,68, 70 and 72, to provide similar compensation.

Referring to FIG. 5, FIG. 5A shows a 150 watt RF driver amplifier of atelevision transmitter and FIG. 5B the compensation signal derivingcircuit for the driver amplifier of FIG. 5A and other amplifiers, all inaccordance with the invention.

Resistor R5, of FIG. 5B, connected via terminals A, in the collectorcircuit of RF driver amplifier transistor V7 of FIG. 5A, (Motorola TypeXPS 1028), acts as the sensor resistor in the internal thermal changecompensation arrangement for transistor V7, and the voltage derivedacross it, varying with the video modulation level and hence the powerlevel handled by the RF driver transistor V7, applied, after the removalof any RF component, to the inverting amplifier OP37. The inverted andvarying DC level from inverting amplifier OP37 is fed to the low passfilter based upon amplifier OP90 which comprises components R10, C6,R11, R12, R13 and C10 whose values, marked upon the circuit diagram, areselected to give a rate of change of bias compensation to transistor V7,related to the internal thermal characteristic of that transistor.

In the circuit shown in FIG. 5B, the rate determining components of thelow pass filter have the values indicated, to provide the appropriaterate of change of bias compensation for V7 for an average collectorcurrent, after filtering the video information, ranging from 1.75 A to4.5 A. The quiescent collector current with no signal input isapproximately 400 mA.

Under high power (black level) conditions, V2 acts to shorten the timeconstant, and below 1.75 A, emitter followers V3/V4 saturate and providean "end-stop" to the action of the circuit.

Modification of these component values would be needed for othertransistors with different internal thermal characteristics.

The bias compensation voltage for the RF driver amplifier is fed to thebias circuit of the driver amplifier via terminals B, and in parallel tothe bias circuits of a number of other RF amplifier stages of thetransmitter, all of which employ the same transistor type as V7 of theRF driver amplifier illustrated in FIG. 5A.

Various modifications of the above-described and illustratedarrangements are possible within the scope of the invention.

For example although the invention has been described with reference totelevision transmitting equipment in which compensation is provided forinternal temperature changes within an amplifying transistor due to RFpower levels varying with the degree of modulation of the signal, theinvention may be applied in other applications where changing RF powerlevels produce changing internal temperature and corresponding changesof conduction point, Vbe.

Also although the invention has been described with specific referenceto a transistor as the amplifying device, it may also be applied tothermionic devices such as amplifying tubes, to correct for similarinternal thermal effects due to changing levels of RF power.

The compensation signal may be derived other than from the main currentpath of the semi-conductor device to which the correction is applied,provided it is derived from a signal which is directly related to theradio frequency signal giving rise to the heating effect within thedevice, for example from the RF power output stage of the signal chainin which the radio-frequency amplifier is incorporated, or from a videosignal amplifier or modulator stage prior to the radio-frequencyamplifier.

We claim:
 1. A radio-frequency amplifier incorporating at least oneamplifying device having an input bias circuit and an output circuitcomprising means for continuously applying to the input bias circuit ofthe device a compensation signal having a magnitude continuously relatedto changes in the radio frequency power handled by the device tocompensate for changes in the conduction point of the device due totemperature changes within the device resulting from changes in theradio frequency power handled by the device in which the compensationsignal related to the changes in the radio frequency power handled bythe device is derived from a sensing element in the output circuit ofthe device.
 2. An amplifier in accordance with claim 1 in which thesensing element is connected in the principal current path of thedevice.
 3. An amplifier in accordance with claim 2 in which the sensingelement is a resistor.
 4. A radio-frequency amplifier incorporating atleast one amplifying device having an input bias circuit comprisingmeans for continuously applying to the input bias circuit of the devicea compensation signal having a magnitude continuously related to changesin the radio-frequency power handled by the device to compensate forchanges in the conduction point; of the device due to temperaturechanges within the device resulting from changes in the radio-frequencypower handled by the device, the compensation signal being derived froma sensing element in the circuit of the device, and in which thecompensation signal is derived from elsewhere in a signal chain of whichthe amplifier .forms a part.
 5. An amplifier in accordance with claim 4in which the compensation signal is derived from a point in the signalchain following the amplifier.
 6. A radio-frequency amplifierincorporating at least one amplifying device, comprising means forapplying to the device a compensation signal related to changes in theradio-frequency power handled by the device to compensate for changes inthe conduction point of the device due to temperature changes within thedevice resulting from changes in the radio frequency power handled bythe device, in which the compensation signal is applied to a biasingcircuit of the device, and wherein the compensation signal is applied tothe biasing circuit of the device by means of a signal filter the timeconstant of which is related to the internal thermal time constant ofthe device due to changes of power level therein.
 7. An amplifier inaccordance with claim 6 in which the time constant of the signal filteris substantially the same as the internal thermal time constant of thedevice.
 8. An amplifier in accordance with claim 2 in which the deviceis a semi-conductor device.